An Iranian-American businessman and his 80-year-old father have each been sentenced to 10 years in prison in Iran for espionage, Tehran's prosecutor said Tuesday.

The Mizan news agency reported Siamak Namazi and his father, Baquer Namazi, both dual citizens, were convicted of "cooperating with the hostile American government."

Siamak Namazi was detained in October 2015 by the Islamic Revolutionary Guard Corps, while visiting his family in Iran. His father, a former UNICEF representative who once served as governor of Iran's Khuzestan province under the U.S.-backed shah, was arrested in February.

The U.S. State Department expressed their deep concern at the convictions Tuesday, saying that Siamak Namazi was unjustly detained.

"We join recent calls by international organizations and U.N. human rights experts for the immediate release of all U.S. citizens unjustly detained in Iran, including Siamak and Baquer Namazi, so that they can return to their families," deputy spokesperson Mark Toner said.

UNICEF expressed its sadness at Baquer Namazi's arrest Tuesday. "He worked tirelessly on behalf of children in all those positions, often in highly difficult circumstances," its statement read. "He deserves a peaceful retirement."

The statement also expressed their concern for Baquer Namazi's health, stating that at his age he should be released "on humanitarian grounds."

Four others were convicted of espionage along with the Namazis, according to the Fars news agency.

Nizar Zaka, a permanent American resident from Lebanon, was among those sentenced to 10 years in prison. He was arrested November 2015 and accused on state television of "numerous deep links to the U.S. military intelligence community."